Get ratio from 2 files in gnuplot

Question

I have 2 dat files:

a.dat
#Xs
100   25
200   56
300   75
400   67


b.dat
#Xs
100   65
200   89
300   102
400   167

I want to draw a graph in the gnuplot where the yy values are a ratio between the values of a.dat and b.dat respectively. e.g., 25/65, 56/89, 75/102, and 67/167.

How I do this? I only know make a plot like this, and not with the ratio.

plot "a.dat" using 1:2 with linespoints notitle
     "b.dat" using 1:2 with linespoints notitle

Answer 1

You cannot combine the data from two different files in a single using statement. You must combine the two files with an external tool.

The easiest way is to use paste:

plot '< paste a.dat b.dat' using 1:($2/$4) with linespoints

For a platform-independent solution you could use e.g. the following python script, which in this case does the same:

"""paste.py: merge lines of two files."""
import sys

if (len(sys.argv) < 3):
    raise RuntimeError('Need two files')

with open(sys.argv[1]) as f1:
    with open(sys.argv[2]) as f2:
        for line in zip(f1, f2):
            print line[0].strip()+' '+line[1],

And then call

plot '< python paste.py a.dat b.dat' using 1:($2/$4) w lp

(see also Gnuplot: plotting the maximum of two files)